1. Field of the Invention
The present invention relates to a gear pump driven by a drive source such as an engine, and more particularly, the present invention relates to a variable displacement gear pump.
2. Description of the Related Art
A discharge flow rate of a gear engagement type pump is determined by a capacity of the pump corresponding to a height, a width of each tooth and the like and rotation speed of a gear (the number of rotations of pump). In the case of using such a gear pump as an oil pump for a vehicle, a capacity of the oil pump depends upon the rotation number of an engine that is a drive source. For this reason, depending upon a driving situation of the vehicle, a discharge flow rate of the oil pump may run short with respect to a flow rate required at a supply portion, whereby a necessary amount of oil cannot be supplied appropriately.
Heretofore, in order to deal with such a problem, variable displacement gear pumps as shown in Japanese Patent Application Publication No. 7-233787, Japanese Patent Application Publication No. 2008-69770, Japanese Patent Application Publication No. 5-79472 and Japanese Patent Application Publication No. 2005-207357 (hereinafter, referred to respectively as “Patent Literatures 1 to 4”) have been proposed. A gear pump disclosed in Patent Literature 1 has an oil-pressure regulating valve for returning part of a discharge rate to a suction side in accordance with discharge pressure of oil to be discharged. This oil-pressure regulating valve allows a capacity of the gear pump (oil pump) to be changed. Further, a gear pump disclosed in Patent Literature 2 adjusts a capacity thereof by changing an engagement width between a drive gear and a driven gear in accordance with the number of rotations of the gear pump. A gear pump disclosed in Patent Literature 3 can change a percentage of a discharge port portion in a fluid discharged space in accordance with a position of a valve, whereby the number of sets of gears corresponding to the percentage of the discharge port portion is changed and a discharge rate can thus be changed. A gear pump disclosed in Patent Literature 4 includes a technique that the rotation number of the gear pump (liquid pump) can be controlled independent of the number of revolutions of an engine.
In ones capable of adjusting a discharge capacity by means of a mechanical structure of the conventional variable displacement pumps as described above, there is a need for addition of mechanisms, such as a plurality of valves, into the gear pump. This leads to complex of a peripheral structure of the gear pump and an increase in the number of parts, and this causes an increase in a product cost thereof. Further, in one having a structure capable of changing the engagement width of the gear, there has been a problem that a mechanism of the gear pump itself becomes complicated and this leads to an increase in the number of parts and growing in size. Further, the more complicated the mechanism of the gear pump becomes, the higher accuracy of dimension the gear pump requires. For this reason, a problem also occurs in durability of the pump. Further, in the pump capable of changing the rotation number to be inputted, there has been a concern to make a control mechanism and procedures for changing the rotation number complicated.